You can set it at 300, render it out as an uncompressed AVI, then import the clip back into Vegas and then set it at 300 again and render back out to an uncompressed AVI and keep doing that untill you get the clip as short as you want it. I know it's probably not the easiest way to do it, but I think Vegas only allows you to go at 300%.
You can do both velocity and playback speed combined to get 12x faster playback. Also, rendering to uncompressed isn't necessary. If you disable resampling then rendering to DV won't suffer any quality loss; every 12th frame will be bit-copied rather than rerendered.
I've used this method several times to compress 93 minutes (a full miniDV tape's worth) of sunset to 30 seconds. It takes three passes at 12x then one last pass at about 3x (trimming the ends a bit) to get the final 30 second length. Definately an impressive video experience to watch!